The present invention relates to processes and apparatus for carrying out liquid-liquid contacting, and more particularly to apparatus whereby such contacting may be carried out continuously.
One example of industrial application of liquid-liquid contacting processes is in liquid-liquid extraction. This entails the mixing of two substantially immiscible liquids, typically an organic and aqueous solution, wherein a component of interest transfers from one of the liquid phases to the other. After the transfer has taken place, the phases are conventionally allowed to separate through a difference in their densities in a settler vessel or other separator vessel. The mixing-settling process must usually be repeated a number of times with counter current flow of the liquids in order to obtain sufficiently complete transfer of the component of interest.
The mixing operation has generally been conducted in mechanically agitated vessels, although mixing in a common pump has been used successfully where the transfer has been sufficiently rapid. The mixed phases are directed to a settler, the size of which is determined by the rate of flow of the liquids and the rate at which the liquid phases disengage. The essential features of the conventional mixer-settlers are shown in U.S. Pat. No. 3,206,288, in the name W. C. Hazen et al, issued Sept. 14, 1965.
The rate of phase disengagement, and hence the size of the settler, is influenced by the drop sizes present in the emulsion that is produced in the mixing process, in particular the fine sizes being slower to disengage.
It has been previously proposed to employ so-called "motionless mixer" elements, which can avoid the formation of fine size droplets, for contacting the two liquid phases together.
One form of the said motionless mixers is described in U.S. Pat. No. 3,286,992 in the name Constantine D. Armeniades et al, issued Nov. 22, 1966. A solvent extraction process employing motionless mixer elements to combine the two liquid phases together has been described in Intermet Bulletin, No. 4, Volume 3, April, 1974, in an article entitled "A New Solvent Extraction System". However, as is described in the article, the flow rate of the liquid through the motionless mixer elements strongly affects the efficiency of the resulting extraction. The higher extraction efficiency that was demonstrated at higher flow velocities is considered to be the result of greater mixing intensities and of the presence of smaller drop sizes in the emulsion at higher liquid velocities. The smaller drops will, however, lead to diminished rate of phase disengagement, and will thus lead to a requirement for larger settler vessels for the separation of liquid phases. In order to obtain an emulsion containing drops of substantially constant size it is essential to feed the two liquids at a constant rate through the motionless mixer apparatus. It is moreover advantageous to provide constant flow rates of the two phases when other kinds of mixer apparatus are employed. An arrangement capable of satisfactorily achieving these constant flow rates does not appear to have been proposed prior to the present invention.
In the industrial processes to which solvent extraction techniques are commonly applied, the sources of supply of the respective liquid phases are of a nature such that each incoming stream of liquid has a flow rate that is subject to variation. The present invention provides means for overcoming the effects of such variation.